This Program Project proposes a fundamental examination of the economic underpinnings of Medicare Part D and its consequences. Part D represents one of the most important reforms to the US Medicare system in its history, and a policy experiment with weighty consequences. We have assembled a team of researchers from several institutions with decades of experience studying the economics of consumer choice, the economics of aging, insurance design, pharmaceutical innovation, and health policy. Centered at RAND, administrative and data cores will promote close collaboration across the research teams, as well as links with CMS, NIA, and the external research community. The data core brings to bear a unique and lengthy list of datasets that permit study of many economic aspects of Part D, and provide opportunities for cross-validation of results. The five projects study Part D from an array of perspectives: beneficiaries; insurers; drug manufacturers; policy; and elderly households. The research projects complement each other in valuable ways, and they are supported by two Cores designed to ensure synergistic effort. Questions about demand for drugs, plan choice, and benefit design will be addressed through both reduced-form and structural analysis, and with a variety of administrative and survey data sets. Basic questions such as whether consumers behave rationally, and the value of Part D, will be addressed. The Administrative Core will support this effort by bringing together a Steering Committee of all project teams and convening annual summer workshops. It will also host two research conferences on Part D. In addition, a pilot program will develop innovative cross-cutting research. The administrative core will also support the development of Part D research in the external research community through visiting scholars, two annual research conferences, public use data files, and postdoctoral positions. In sum, this program project will provide an integrated economic evaluation of the welfare effects of Part D, and promote Medicare-related research of policy and scientific interest.